If your 90hp is a 1998 model, you're not achieving recommended max WOT rpm with your current prop. And if it's newer than that, definitely overpropped. You're pushing a heavy boat, and it's better to prop close to the high-end of the WOT recommendation in that case.

The same pitch in stainless would probably work out great on a '98 motor, but if you have the '99 or '00 model, you might want to drop to a 14" pitch.

This would increase your hole shot, bring the WOT rpm up into the recommended range, and probably increase boat speed.

I found the below info at a Solas prop site:

90 Ficht 1998 4500-5500 rpm
90 Ficht 1999-2000 5500-6000 rpm

HTH.........ed

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Talked with online prop shop at some length and they would not recommend a SS on a 98 Ficht as there is concern that something in the drive train is a little week to swing the heavier SS over the equivalent Aluminum. In addition since I'm only 200rpm off the max recommended 5300 vs 5500 in their opinion I was pretty much all in and although they would sell me anything I wanted they did not recommend any changes.

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We in Florida need to get you into our Florida Glasspar Club, which myself and two other guys formed 12 years ago!! We now have 150 or so Glasspars of all makes and models in our club. All you need to do to join is own a Glasspar!!! No politics, no dues, etc. Just get together several times a year and play with our Glasspars in the Water!!! We also have our own website with chat and a lot of good information about Glasspar boats!!
We have a mid-west chapter now! Where do you live?????

WE HAVE SEVERAL SEDANS in the club and you can get much info about performance, restoration, engines, etc. on them!!

I think your 85 should run faster. My brother in law had a 15 ft Glastron tri hull with the same motor you have and it ran low mid 40s. I now have that motor and it runs 40 on a 17 ft boat. Are you sure it's hitting on all 4 cylinders?

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Jim I tried to join your group even though I'm about as far away as you can get, Fresno CA. Since in addition to the 2 Seafair Sedans, Avalon and G3 I also have a couple of finned vessels, Redfish and Glass Craft. Anyhow can't remember what happened but for some reason I was unable to complete the log in. Will try it again.

Will be messing with the prop issue again when I finish the 2nd Seafair as I have a 2012 90hp Etec with 50 hrs I picked up the other day on a really derelict Seafair Sunliner. Boat is so bad that I will be scrapping it and I'm not big into that.

Visiting for a Glasspar get together is not out of the question as I drove to Kissimmee to race sailboats back in the 80's.
Jack

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In all likelihood the aluminum prop is a generic or OMC "universal" prop. The issues are thick'ish blades, not really effective cup on trailing edge, etc.

Think about a prop slicing into the water, the thinner the blade, the easier it is to do. SS props that are well made will have about 1/2 the blade thickness you have in the average aluminum prop.

In addition you can knock the dings out of a SS prop many times and file the edges sharp and true. Not something you can do more than about once with an aluminum prop. So I'm partial to Michigan Wheel props, but there are many good ones.

So next question is where is the cavitation plate in relationship to the bottom? If your bottom is fair and smooth, you can run the cav plate about 1/2" above the bottom (at least) and that will reduce your lower unit drag by a fair amount.

Look over the transom when on plane. See that water "coming up" as it leaves the trailing edge of the hull... You want that rising stream to just hit the bottom of the cav plate. The cav plate is there so you do not pull air onto the prop tips. But the 13.25 is a fair ways down from the plate. If the rising stream is just washing the bottom of the cav plate, you are well away from drawing air

OK, put all this together. Thinner sharper prop. Raise the lower unit to reduce drag. Better cup to push harder at max RPM. It all means more speed and better fuel economy at cruise.

Get a nice used SS prop off eBay and try it out. If it's an improvement, then consider going to a 4-blade. That will add stern lift so you can trim out a bit further and it'll still ride right. That will further reduce drag and you'll pick up a bit more.

But, you need to have a good running SS 3-blade so you can tell them exactly what it's doing before they can offer the correct 4-blade ...

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Ed, just what is WOT speed for an 85hp Evinrude pushing a 15' tri hull? Right now rpm are slightly over 5G and getting about 32/33 mph GPS.

This thread's a little old, but I just saw it. My 15' tri hull with the 85hp Rude runs an indicated 30mph by the speedo. I haven't checked it by GPS at top end, but it's close when slower. I don't have a tach, so who knows on RPMs. I feel like it should be faster, but I'm still dialing in the manual trim.

btw, I that's with a 13-1/2" x 17 aluminum 3 blade prop.

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